How many police shootings a year? No one knows

People march in Washington on Sept. 6 to protest the killing of Michael Brown. (Nicholas Kamm/AFP/Getty Images)

A summer of high-profile police shootings, most notably the Aug. 9 shooting of 18-year-old Michael Brown in Ferguson, Mo., has rekindled a decades-long debate over law enforcement’s use of lethal force.

Police unions and some law-and-order conservatives insist that shootings by officers are rare and even more rarely unjustified. Civil rights groups and some on the left have just as quickly prescribed racial motives to the shootings, declaring that black and brown men are being “executed” by officers.